Taking a taxi ride in Penang is very convenient to get from one point to another point, however it could be quite expensive compared bus.
Even tough the state government had enforced all taxi in the city to use meter, not many will adhere to it.
However, they have a common taxi rate. For short distances it is between is about RM 3 to RM 6. From Batu Ferringhi to Georgetown it will cost about RM 30. Whereas from Batu Ferringhi to the airport it will cost about RM 60.
Ride Rapid Penang Bus, if you're not in a hurry.
Useful Taxi Numbers